1. Field of the Invention
The present invention relates to a switching amplifier, and more particularly, to an amplifier for generating compensating current via a feedback circuit to keep stability of a feedback system, thereby responding to the change of input signals immediately to decrease output distortion.
2. Description of the Prior Art
Power amplifiers are utilized for amplifying signal power, and are usually classified into A, B, AB, C and D types. The D type amplifier adopts a Pulse-Width Modulation (PWM) technology, which adjusts duty cycles of square waves to represent input values. When the D type amplifier works, an output-stage transistor of the D type amplifier transforms from extremely high impedance to extremely low impedance. The output-stage power loss of the D type amplifier is less than those of other amplifiers due to a short period in an active region, so that the D type amplifier has higher efficiency and is utilized in an audio output system.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a D type amplifier 10 in the prior art. The D type amplifier 10 comprises an integrator 100, a comparator 102, and a driving circuit 104. The integrator 100 is a two-order integrator utilized for performing a two-order integration operation on an input signal VI, and comprises operational amplifiers 106, 108, resistors 110, 112, 114, 116, and capacitors 118, 120. The resistors 112, 114, 116 and the capacitors 118, 120 determine a bandwidth of the D type amplifier 10, and the resistors 110, 112 determine the gain of the D type amplifier 10. The comparator 102 is coupled to an output end of the integrator 100 for comparing an output signal of the integrator 100 and a triangle signal 122, and outputs a comparison result to the driving circuit 104. Then, the driving circuit 104 outputs an output signal VO to a load circuit (not shown in FIG. 1) and feedbacks the output signal VO to an input end of the integrator 100 according to the comparison result of the comparator 102.
Please refer to FIG. 2, which illustrates a schematic diagram of related signal waveforms when the D type amplifier 10 shown in FIG. 1 operates. In FIG. 2, WID represents an ideal waveform of the D type amplifier 10 (after passing a low-pass filter), WVO represents a real output waveform of the D type amplifier 10 (namely, the waveform of the output signal VO), WVIO represents a waveform of an output signal VIO of the integrator 100, WTRI represents a waveform of the triangle signal 122, and WVI2 represents a waveform of an input signal VI2 of the operational amplifier 108. Moreover, because of system constraint, VO_max represents the maximum possible value of the output signal VO, and VIO_max represents the maximum possible value of the output signal VIO of the integrator 100. Thus, as shown in FIG. 2, when the output level of the D type amplifier 10 reaches the maximum value VO_max, the output level of the integrator 100 exceeds the triangle signal 122, causing the output of the D type amplifier 10 to be saturated. In such a situation, if the output level of the integrator 100 reaches the maximum VIO_max, a charging current flowing through the capacitor 120 will lower the level of the input signal VI2, causing the operational amplifier 108 to operate in an open loop state. Then, if the system needs to lower the output signal VO, the prior art needs to discharge the charge in the capacitor 120 to recover the state of the operational amplifier 108 back to a close loop state, which wastes extra time. However, the extra discharging time causes the output of the D type amplifier 10 to deviate from the ideal state, and generates distortion for the audio output system.